parsebin: Post STREAM_COLLECTION on EVENT_STREAM_COLLECTION

Otherwise decodebin won't get notified about STREAM_COLLECTION comming
from the sources and thus will never get informored about it. Without
being informed about the stream collection decodebin won't be able to
select any streams. It ends up not creating any output for the streams
defined from outside parserbin.
https://bugzilla.gnome.org/show_bug.cgi?id=795364